1. Field of the Invention
The present disclosure relates to an interference measurement method and apparatus for use in a Distributed Antenna System (DAS).
2. Description of the Related Art
The mobile communication system has evolved into a high-speed, high-quality wireless packet data communication system to provide data and multimedia services beyond the early voice-oriented services. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and LTE-Advanced (LTE-A) defined in 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined in IEEE, have been developed to support the high-speed, high-quality wireless packet data communication services. Particularly, LTE a communication standard developed to support high speed packet data transmission and to maximize the throughput of the radio communication system with various radio access technologies. LTE-A is the evolved version of LTE to improve the data transmission capability.
LTE is characterized by 3GPP Release 8 or 9 capable base station and terminal (user equipment) while LTE-A is characterized by 3GPP Release 10 capable base station and user equipment. As a key standardization organization, 3GPP continues standardization of the next release for more improved performance beyond LTE-A.
The existing 3rd and 4th generation wireless packet data communication systems (such as HSDPA, HSUPA, HRPD, and LTE/LTE-A) adopt Adaptive Modulation and Coding (AMC) and Channel-Sensitive Scheduling techniques to improve the transmission efficiency. AMC allows the transmitter to adjust the data amount to be transmitted according to the channel condition. That is, the transmitter is capable of decreasing the data transmission amount for bad channel condition so as to fix the received signal error probability at a certain level or increasing the data transmission amount for good channel condition so as to transmit large amount of information efficiently while maintaining the received signal error probability at an intended level. Meanwhile, the channel sensitive scheduling allows the transmitter to serve the user having good channel condition selectively among a plurality of users so as to increase the system capacity as compared to allocating a channel fixedly to serve a single user. This increase in system capacity is referred to as multi-user diversity gain. In brief, the AMC method and the channel-sensitive scheduling method are methods for receiving partial channel state information being fed back from a receiver, and applying an appropriate modulation and coding technique at the most efficient time determined depending on the received partial channel state information.
In case of using AMC along with Multiple Input Multiple Output (MIMO) transmission scheme, it may be necessary to take a number of spatial layers and ranks for transmitting signals in to consideration. In this case, the transmitter determines the optimal data rate in consideration of the number of layers for use in MIMO transmission.
Recently, many researches are being conducted to replace Code Division Multiple Access (CDMA) used in the legacy 2nd and 3rd mobile communication systems with Orthogonal Frequency Division Multiple Access (OFDMA) for the next generation mobile communication system. The 3GPP and 3GPP2 are in the middle of the standardization of OFDMA-based evolved system. OFDMA is expected to provide superior system throughput as compared to the CDMA. One of the main factors that allow OFDMA to increase system throughput is the frequency domain scheduling capability. As channel sensitive scheduling increases the system capacity using the time-varying channel characteristic, OFDM can be used to obtain more capacity gain using the frequency-varying channel characteristic.
FIG. 1 is a graph illustrating a relationship between time and frequency resources in LTE/LTE-A system.
As shown in FIG. 1, the radio resource for transmission from the evolved Node B (eNB) to a User Equipment (UE) is divided into Resource Blocks (RBs) in the frequency domain and subframes in the time domain. In the LTE/LTE-A system, an RB consists of 12 consecutive carriers and occupies 180 kHz bandwidth in general.
Meanwhile, a subframe consists of 14 OFDM symbols and spans 1 msec. The LTE/LTE-A system allocates resources for scheduling in unit subframe in the time domain and in unit of RB in the frequency domain.
FIG. 2 is a time-frequency grid illustrating a single resource block of a downlink subframe as a smallest scheduling unit in the LTE/LTE-A system.
As shown in FIG. 2, the radio resource is of one subframe in the time domain and one RB in the frequency domain. The radio resource consists of 12 subcarriers in the frequency domain and 14 OFDM symbols in the time domain, i.e. 168 unique frequency-time positions. In LTE/LTE-A, each frequency-time position is referred to as Resource Element (RE).
The radio resource structured as shown in FIG. 2 can be used for transmitting plural different types of signals as follows.                CRS (Cell-specific Reference Signal): reference signal transmitted to all the UEs within a cell        DMRS (Demodulation Reference Signal): reference signal transmitted to a specific UE        PDSCH (Physical Downlink Shared Channel): data channel transmitted in downlink which the eNB use to transmit data to the UE and mapped to REs not used for reference signal transmission in data region of FIG. 2        CSI-RS (Channel Status Information Reference Signal): reference signal transmitted to the UEs within a cell and used for channel state measurement. Multiple CSI-RSs can be transmitted within a cell.        Other control channels (PHICH, PCFICH, PDCCH): channels for providing control channel necessary for the UE to receive PDCCH and transmitting ACK/NACK of HARQ operation for uplink data transmission        
In addition to the above signals, zero power CSI-RS can be configured in order for the UEs within the corresponding cells to receive the CSI-RSs transmitted by different eNBs in the LTE-A system. The zero power CSI-RS (muting) can be mapped to the positions designated for CSI-RS, and the UE receives the traffic signal skipping the corresponding radio resource in general. In the LTE-A system, the zero power CSI-RS is referred to as muting. The zero power CSI-RS (muting) by nature is mapped to the CSI-RS position without transmission power allocation.
In FIG. 2, the CSI-RS can be transmitted at some of the positions marked by A, B, C, D, E, F, G, H, I, and J according to the number of number of antennas transmitting CSI-RS. Also, the zero power CSI-RS (muting) can be mapped to some of the positions A, B, C, D, E, F, G, H, I, and J. The CSI-RS can be mapped to 2, 4, or 8 REs according to the number of the antenna ports for transmission. For two antenna ports, half of a specific pattern is used for CSI-RS transmission; for four antenna ports, entire of the specific pattern is used for CSI-RS transmission; and for eight antenna ports, two patterns are used for CSI-RS transmission. Meanwhile, muting is always performed by pattern. That is, although the muting may be applied to plural patterns, if the muting positions mismatch CSI-RS positions, it cannot be applied to one pattern partially.
In a cellular system, the reference signal has to be transmitted for downlink channel state measurement. In the case of the 3GPP LTE-A system, the UE measures the channel state with the eNB using the CSI-RS transmitted by the eNB. The channel state is measured in consideration of a few factors including downlink interference. The downlink interference includes the interference caused by the antennas of neighbor eNBs and thermal noise that are important in determining the downlink channel condition. For example, in the case that the eNB with one transmit antenna transmits the reference signal to the UE with one receive antenna, the UE has to determine energy per symbol that can be received in downlink and interference amount that may be received for the duration of receiving the corresponding symbol to calculate Es/Io from the received reference signal. The calculated Es/Io is reported to the eNB such that the eNB determines the downlink data rate for the UE.
In the typical mobile communication system, the base station apparatus is positioned at the center of each cell and communicates with the UE using one or plural antennas deployed at a restricted position. Such a mobile communication system implemented with the antennas deployed at the same position within the cell is referred to as Centralized Antenna System (CAS). In contrast, the mobile communication system implemented with plural Remote Radio Heads (RRHs) belonging to a cell are distributed within the cell area is referred to as Distributed Antenna System (DAS).
FIG. 3 is a diagram illustrating an exemplary antenna arrangement in the conventional distributed antenna system.
In FIG. 3, there are distributed antenna system-based cells 300 and 310. The cell 300 includes five antennas including one high power transmission antenna 320 and four low power antennas 341, 342, 344, and 343. The high power transmission antenna 320 is capable of providing at least minimum service within the coverage area of the cell while the low power antennas 341, 342, 343, and 344 are capable of providing UEs with the high data rate service within a restricted area. The low and high power transmission antennas are all connected to the central controller and operate in accordance with the scheduling and radio resource allocation of the central controller. In the distributed antenna system, one or more antennas may be deployed at one geometrically separated antenna position. In the distributed antenna system, the antenna(s) deployed at the same position is referred to as Remote Radio Head (RRH).
In the distributed antenna system depicted in FIG. 3, the UE receives signals from one geometrically distributed antenna group and regards the signals from other antenna groups as interference.
FIG. 4 is a diagram illustrating an exemplary situation of interference between antenna groups transmitting different UEs in the conventional distributed antenna system.
In FIG. 4, the UE1 400 is receiving traffic signal from the antenna group 410. Meanwhile, the UE2 420, UE3 440, and UE4 460 are receiving traffic signals from antenna groups 430, 450, and 460, respectively. The UE1 400 which is receiving the traffic signal from the antenna group 410 is influenced by the interference of the other antenna groups transmitting traffic signals to other UEs. That is, the signals transmitted the antenna groups 430, 450, and 470 cause interferences to UE1 400.
Typically, in the distributed antenna system, the interferences caused by other antenna groups are classified into two categories:
Inter-cell interference: interference caused by antenna groups of other cells
Intra-cell interference: interference caused by antenna groups of same cell
In FIG. 4, the UE 1 undergoes intra-cell interference from the antenna group 430 of the same cell and inter-cell interference from the antenna groups 450 and 47 of a neighbor cell. The inter-cell interference and the intra-call interference are influence the data channel reception of the UE simultaneously.
In order for the DAS-capable UE to receive downlink signal at optimal data rate, it is necessary to measure the inter-cell interference and intra-cell interference accurately and compare these with the received signal strength to request the eNB for the data rate based on the comparison result.
Unlike DAS, Centralized Antenna System (CAS) has only one antenna group. In this case, there is on intra-cell interference caused by other antenna groups within the same cell but inter-cell interference caused by the antenna groups of neighbor cells. In the case that the LTE/LTE-A system is implemented based on CAS, it is possible to measure the inter-cell interference using the CRS described with reference to FIG. 2. Typically, in the DAS-based system, the UE performs Inverse Fast Fourier Transform (IFFT) on the CRS having periodic characteristic in frequency domain to generate delay domain signal.
FIG. 5 is a graph illustrating delay domain signals converted from CRS.
In the LTE/LTE-A system, if the signal is converted to delay domain single through IFFT, it is possible to acquire the channel impulse response having the tendency in which the energy carried by the delay component decreases as the delay increases as shown in FIG. 5. Typically, the tail part of the signal acquired through IFFT corresponds to the interference caused by other cell while head part of the signal corresponds to the actual signal component of CRS. In this case, the UE is capable of calculating Signal to Noise ratio by measuring the size of the interference at the tail part. Such an interference measurement is possible because different cells transmit no same CRS. Since the different cells transmit CRSs using different frequency-time resources and the cells apply unique scrambling codes, the above interference measurement is possible. In the case of LTE/LTE-A, the scrambling of the CRS is determined by Cell ID of the corresponding cell.
In the DAS-based LTE/LTE-A system, however, all antenna groups of the same cell transmit the CRS at the same timing and cannot apply unique scrambling CRSs. If the different antenna groups of the same cell cannot transmit unique CRSs, although the inter-cell interference amount from the antenna groups of the neighbor eNBs can be measured, it is impossible to measure the intra-cell interference from other antenna groups of the same cell.
In the case of measuring the interference amount using the method described with reference to FIG. 5, the UE is capable of calculating the interference caused by the antenna groups of other cells but not the interference caused by the other antenna groups of the same cell, resulting in inaccurate Signal-to-Interference ratio. The inaccurate Signal-to-Interference ratio causes significant performance degradation of the LTE/LTE-A system which determines downlink data rate using AMC based on the Signal-to-Interference ratio.
The present disclosure proposes an efficient interference measurement method and apparatus to solve this problem.
As described above, in order to determine the downlink data rate efficiently in the DAS-based communication system, the UE has to have the capability of measuring the intra-cell interference as well as the inter-cell interference. In order to accomplish this, the present disclosure proposes a method for measuring interference based on CSI-RS.